What in the world are ‘phylacteries”?

I have always wondered why tefillin are referred to as ‘phylateries’. I found the answer on aish.com.

Tefillin are sometimes referred to as phylacteries. This stems from the ancient Greek phylakterion, which means a safeguard. Apparently, the Greeks misunderstood the Tefillin to be some sort of amulet or charm. Actually, Tefillin serve not as a superstition, but as a bona fide connection to God.